Entomophthoromycotina Harpellomycotina

- BLASTOCLADIOMYCOTINA (treated with chytrids here)

Basidiomycota

Glomeromycota Zygomycota

Chytridiomycota

FIGURE 3.1 Relationships of major fungal groups. (Modified from J. Taylor and Berbee, 2006.)

are that more than 1.5 million species are yet to be discovered (Hawksworth et al., 1995). Other organisms that were historically included with the fungi, but are now placed in the Kingdom Chromista, include the Oomycetes (water molds), Hyphochytridiomycetes, and Labyrinthulomycetes. Of these three clades, only the water molds will be discussed here. Slime molds were also once considered to be Fungi, but are now included in the Kingdom Protista. The fossil record of slime molds (FIGS. 3.2-3.4), once called the myxomycetes, is meager, restricted only to several found in Eocene amber (Dorfelt and Schmidt, 2006), so they will not be covered in this book.

Together with a few other groups of heterotrophic organisms (e.g., bacteria), the fungi are the principal decomposers in the biosphere, releasing CO2 into the atmosphere and nitrogenous compounds into the soil. They also function as bioweathering agents and transformers of minerals and rocks (Burford et al., 2003). Fungi are involved in numerous types of associations with other organisms, ranging from those that produce diseases in plants and animals to a variety of beneficial, symbiotic relationships (e.g., mycorrhizae in the roots of most vascular plants) (Newsharn et al., 1995). Despite the fact that fungi, as a group, have a long and interesting geologic history (Tiffney and Barghoorn, 1974), only relatively recently have they been studied in any detail. As a result, their importance in the evolution of past biotas and our knowledge of the evolutionary history of the major fungal groups through time have been minimized. There are several reasons why our understanding of fossil fungi has lagged behind our knowledge of many other fossil organisms. One of these is the long-held belief that fungi are too fragile to be adequately preserved in the fossil record (FIG. 3.5 ) . Additionally, the study of fossil fungi may have been avoided due to difficulties in recognizing and interpreting them (FIGS. 3.6, 3.7). This is especially true in situations where fungi have been found within permineralized vascular plants (FIG. 3.8), which typically represent the principal focus of the research (LePage et al., 1994). In addition, when collecting fossil plants, paleobotanists may introduce their own inherent bias by focusing on collecting the best specimens of a taxon (e.g., those that provide the most informative characters of the organism). As a result, specimens that might show the effects of fungal activities, such as degraded tissue or necrotic areas, are simply not brought back to the laboratory for study. Some of this bias has been overcome by the modern use of

FIGURE 3.2 Arcyria sulcata (slime molds), sporocarp composed of a stalk and cupuliform base of the pteridium (cup) (Eocene). Bar = 100 pm. (Courtesy A. Schmidt.)

quantitative field techniques, in which all available material within a certain area is examined or collected. Finally, the environment of deposition and fossilization may affect preservation. For example, Carboniferous swamp plants often occur in peat deposits, and the chemistry in the swamp may have discouraged extensive fungal activity. Nevertheless, despite all of these limitations, the literature on fossil fungi and their biotic and abiotic interactions is rapidly increasing. As might be expected, some of the details, especially those that characterize levels of fungal interaction in ecosystems (Taylor, 1990; Taylor et al., 2004) are more difficult to determine from fossils.

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